1. Field of the Invention
The present invention relates to a data processing apparatus comprising two magnetic disk units of the same type which store identical data and serve as backup disk units.
2. Description of the Related Art
A data processing apparatus of a dual disk computer system is known. This apparatus uses two magnetic disk units of the same type as backup means and causes them to store identical data. In such a conventional apparatus, input/output (I/O) control of these two magnetic disks are performed by the following two methods. According to the first method, one magnetic disk unit is used exclusively as a backup unit while read request channel programs are permanently chained in the I/O queue of the other magnetic disk unit. According to the second embodiment, each read request channel program is chained in the I/O queue of one of the first and second magnetic disk units which has a smaller I/O queuing count.
The I/O execution time of a magnetic disk unit greatly depends on a seek width on the magnetic disk. More specifically, the I/O time of a disk is given by: EQU D=S+R+T
where S is the seek time, R is the rotational wait time, and T is the transfer time. Of these times, the seek time is the longest. It is most important to minimize the seek time.
In both the first and second methods, however, no consideration is given to the seek width. Therefore, I/O processing cannot be performed at high speed.